TOPOGRAPHY OF GLACIERS AND ICE CAPS WITH SPOT 5 HRS
Jérôme Korona a *, Etienne Berthier b, Marc Bernard a, Frédérique Rémy b, Eric Thouvenot c
a
Spot Image, 5 rue des satellites, BP 14 359, 31030 Toulouse cedex 4, France (Jerome.Korona, Marc.Bernard)@spotimage.fr
b
OMP-LEGOS, 14 avenue Edouard Belin, 31400 Toulouse, France (etienne.berthier, Frederique.Remy)@legos.obs-mip.fr
c
CNES, 18 avenue Edouard Belin, 31401 Toulouse cedex 9, France - eric.thouvenot@cnes.fr
Commission VIII, SS–12
KEY WORDS: DTM, Spot Image, Space photogrammetry, Spatial modelling, Glaciology, International Polar Year, Stereoscopic,
Archiving
ABSTRACT:
Monitoring the evolution of glaciers, ice caps and ice streams in polar areas is of outmost importance because they constitute a good
indicator of global climate change and contribute significantly to ongoing sea level rise. Accurate topographic surveys are crucial as
they reflect the geometric evolution of ice masses. Unfortunately, the precision and/or spatial coverage of available data from
satellite missions (radar altimetry, ICESat) or field surveys is generally insufficient.In 2006, a pilot project led by Spot Image and
IGN showed that SPOT 5 stereoscopic pairs could provide 40m Digital Terrain Models of the Antarctic Peninsula and Alaska within
an absolute horizontal precision of 30m RMS.In 2007, the French Space Center (CNES) decided, within the framework of the
International Polar Year and the GIIPSY project, to fund the SPIRIT (SPOT 5 stereoscopic survey of Polar Ice: Reference Images
and Topographies) project, a huge HRS coverage of polar areas. Thanks to this program, jointly managed by Spot Image and the
LEGOS, the opportunity is given to the Scientific Community to browse a massive archive of stereoscopic pairs and to freely obtain
large amounts of DTMs over the polar areas. This paper will present the stereoscopic coverage achieved so far over Northern and
Southern polar areas up to 81°N/S. We will also describe in details the Polar DALI web interface and the specific SPIRIT product.
The conclusion will summarize the impact of the availability of such high accuracy elevation data on glaciology research.
knowledge is also crucial for accurate dating of ice cores
(Parrenin et al., 2004).
1. INTRODUCTION
During the last two decades, the cryosphere has been the theatre
of rapid and major changes. Shrinkage of mountains glaciers
and ice caps have accelerated during the past ten years, with a
contribution to sea level rise growing from 0.33 mm per year
(for the 1961-1990 period) to 0.8 mm per year (for the 20012004 period) (Kaser et al., 2006). Break up of Larsen A and B
ice shelves in the Antarctic Peninsula have led to the thinning
and acceleration of the glaciers located upstream (De Angelis
and Skvarca, 2003; Rott et al., 1996; Scambos et al., 2004).
Major changes in the ice dynamics have also been recently
detected in Greenland, leading to rapid ice loss (Howat et al.,
2007; Joughin et al., 2004; Rignot and Kanagaratnam, 2006).
Thus, the cryosphere appears as one of the major actor and
indicator of ongoing climate change (IPCC International Panel
on Climate Change, 2007).
Yet, the topography of glaciers, ice caps, ice shelves of both
polar areas remains poorly known. Obtaining a homogenous
and precise topography of these remote regions is a top-priority
task to understand and characterize their reaction to recent
climate change and their contribution to ongoing ocean level
rise (Cazenave, 2006). A reference and comprehensive
topography of these regions is also necessary to detect future
evolutions. Furthermore, as the topography of ice masses is a
free surface resulting of external forcings and physical
processes within the ice (Remy et al., 1999), it can be used to
test ice dynamic models or as an initial condition to predict
their evolution (Remy and Parrenin, 2004; Ritz et al., 2001). Its
The topography of polar ice masses is still poorly known
because in situ observations are difficult and sparse. Spaceborne measurements of the ice topography are also difficult and
not always a priority. For example, Polar Regions were not
surveyed by the Shuttle Radar Topography Mission (SRTM) in
February 2000 (Rabus et al., 2003). Satellite radar altimeters
on-board ERS1, ERS2 and Envisat have the capability to
measure the surface elevation of large ice masses with a
resolution of about 1 km and a good relative precision as soon
as the slopes are gentle (Legresy et al., 2005; Shepherd and
Wingham, 2007). But this technique is not effective for steeper
areas and, thus, cannot accurately map the coastal regions of the
two ice sheets or most mountains glaciers and ice caps. The
GLAS (Geosciences Laser Altimeter System) instrument on
ICESat surveys altimetric profiles with a laser footprint of 65 m
every 170 m along track. Its revolution period lasts 183 days,
which implies a distance between two consecutives tracks
varying from 2.5 km at latitude of 80° to 15 km at the equator.
GLAS provides very accurate (+/- 10 cm) but sparse
measurements of ice surface elevation.
Despite the strong albedo and the lack of texture of snow or ice,
stereoscopic optical images have already proved to be a
valuable mean to obtain large-scale topographies of ice masses
(Berthier et al., 2004; Stearns and Hamilton, 2007). In
particular, good results have already been obtained over Alaska
icefields and the Antarctic Peninsula during preliminary studies
* Corresponding author
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using the HRS sensors on-board SPOT 5 (Berthier and Toutin,
2008; Durand, 2006). Following these promising results and to
contribute to the International Polar Year (IPY), the French
space agency (CNES), Spot Image and the LEGOS (Laboratory
Studying Geophysics and Oceanography from Space) decided
to launch the SPIRIT (SPOT 5 stereoscopic survey of Polar Ice:
Reference Images and Topographies) project. The goal of the
present article is to describe the SPIRIT project and
demonstrate the high glaciological potential of the SPIRIT data.
2. THE SPIRIT INTERNATIONAL POLAR YEAR
PROJECT
The main aims of the SPIRIT project are (1) to build a
comprehensive archive of polar ice based on SPOT 5 HRS
images and (2), for selected regions, to produce DTMs and
ortho-images that will be delivered for free to the scientific
community involved in IPY projects. In this section, we will
first present the SPOT 5 HRS sensors. We will then describe
the target areas and the acquisitions obtained so far (as of 17
April 2008) before presenting the Polar DALI web interface
designed to browse the archive. At the end of this section we
will provide the characteristics of the SPIRIT product (DTMs
and ortho-image) that is going to be delivered to the scientific
community.
2.1 The HRS sensors onboard SPOT 5
The HRS sensors, embedded on SPOT 5, were designed for
DTM generation by acquiring pairs of images in a single pass of
the satellite. It is composed of two telescopes, respectively
pointing with an angle of 20° rear and 20° front from the
satellite vertical, providing a base-to-height ratio of 0.8. A 600
km by 120 km stereoscopic pair is captured within 180 seconds
(Fig. 1), with a ground resolution of 5 m along track by 10 m
across track. The acquisition mode is panchromatic (0.48 µm 0.71 µm).
data (good high frequency accuracy), the final absolute location
precision is 30 m rms.
A more detail description of the SPOT 5 HRS satellite mission
can be found elsewhere (Bouillon et al., 2006).
2.2 Building a SPOT 5 HRS archive over polar ice masses.
The first step of the SPIRIT project was the selection of target
areas and the definition of their relative priority by the LEGOS
(Principal Investigator) in the framework of the GIIPSY (Global
Interagency IPY Polar Snapshot Year) project. One major
constrain was the 81.15° North – 81.15° South acquisition
limits of the SPOT 5 orbit. The flat, snow-covered and
homogenous central regions of the Antarctic and Greenland ice
sheets were deliberately excluded of the target areas because
DTM derived from stereoscopic optical images would not reach
the centimetric accuracy already obtained using radar or laser
altimetric surveys.
Thus, three majors groups of target areas were considered: the
coastal regions of Antarctica, the margins of the Greenland ice
sheet and small glaciers and ice caps that surround the Artic
ocean and Antarctica. Overall, these three groups encompass
more than 2.5 millions square kilometers. These 108 target
areas were classified using three different priority degrees (Fig.
2 and 3):
Top Priority: Very sensitive areas in Greenland, Antarctica and
fast changing ice caps and icefields (Pine Island Glacier,
Jakobshavn Glacier, Patagonian Icefields, Vatnajökull ice cap
etc.).
Important Priority: All other major ice streams of the two
inlandsis, draining most of the snow of the accumulation areas,
whose dynamics is crucial for the mass balance of the polar ice
sheets. Identification of all these major outlet ice streams was
possible by the mean of SAR velocity mosaics (Rignot, 2006;
Rignot and Kanagaratnam, 2006) and also two image mosaics
derived from MODIS data (Scambos et al., 2007). This medium
priority level encompasses also some small glaciers and ice
caps where fast evolutions have been reported (Alaska, Iceland)
or poorly known ones such as Vilchek land Ice cap, Franz Josef
Land (Dowdeswell, personal communication).
Standard Priority: Remaining areas at the periphery of the
Greenland and Antarctic ice sheets and the other ice-covered
regions close to the poles of both hemispheres.
Figure 1. SPOT 5 HRS sensors acquisition process
The relative precision and the radiometric homogeneity are
enhanced by the simultaneity of the acquisitions, implying that
the textures, illumination, orbital and altitude parameters are the
same for both images. Over ice-free landmasses, HRS provides
DTMs with a vertical accuracy of 10 m with 90% confidence
on slopes less than 20% (Bouillon et al., 2006).
The priority levels can be changed in the course of IPY if
needed, in case of spectacular events (ice-shelf break-up for
example). Few areas can be acquired twice in order to study
their quick (one year) evolution. The Antarctic Peninsula has
already been surveyed by SPOT 5 HRS in 2006 and will
probably be re-surveyed during the second year of IPY in 2009.
This is why it was not included in the first southern hemisphere
acquisition campaign.
Finally, thanks to the onboard stellar location unit, using startracker data (good low frequency accuracy) and gyroscopes
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Discriminating snow from cloud is really a tough task, almost
impossible for the current automatic algorithms, and thus, must
be performed by a human operator. To optimize the acquisition
campaign, cloud detection needs to be performed on a daily
basis. Indeed, once a target region has been covered by cloud
free images, it has to be immediately excluded from the satellite
schedule so that the satellite resource is devoted to other
programming tasks.
Another key issue is the appropriate setting of the HRS sensor
gains. The generation of DTM from optical stereo-images is
only possible if the radiometric range of the images is sufficient
to find homologous points using a correlation window, which
implies that the gain must not be too high (images would be
saturated) nor too low (too limited radiometric range).
Considering a snow-covered Earth surface (albedo of about 0.9),
the best sensor gain is evolving depending on the time of year
and the latitude (Raup et al., 2000). Consequently, the gain
setting is modified on a weekly basis for each areas of interest
in order to insure the best correlation score during the DTM
production phase.
The cloud-free coverage achieved during the summer 2007
campaign in the northern hemisphere is presented in Fig. 4.
About 44% of the total arctic targeted surface (~ 300 000 km²)
was covered between 1 July and late October 2007.
Figure 2. 43 areas of interest delimited in the Northern
hemisphere. They cover about 680000km² (see details in Table
6).
Figure 3. 65 areas of interest retained in the Southern
hemisphere. They cover about 1900000km² (see details in Table
7).
The SPIRIT project started the 1 July 2007 and SPOT 5 HRS
images will be acquired until 30 April 2009. The acquisition
periods for each hemisphere have been chosen to cope with the
polar night: June to November for Northern areas, December to
April for Southern areas, with local variations according to
latitude.
The nominal revisiting period of HRS, considering its alongtrack acquisition mode, equals 26 days. However, the orbit
tightening at high latitudes leads to a briefer delay (sometimes 1
or 2 days) between two acquisitions. Despite some difficult
weather conditions, this explains why a significant coverage has
already been achieved during the first months over both
hemispheres.
Figure 4. North hemisphere Spot5 HRS coverage during the
2007 acquisition campaign.
The ongoing Southern campaign (December 2007 to April 2008)
is even more successful (Fig. 5). As of 17 April 2008, 77.21%
of the targeted areas have already been covered (about 1 480
000 km²). This is due to the absence of conflicts in the daily use
of SPOT 5.
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online consultation on the project dedicated interface: Polar
DALI.
2.3 The Polar DALI Interface
The SPIRIT quick-looks (HRS images in lower resolution: 120
meters) can be browsed through the Polar DALI interface. The
access is possible through a valid login, delivered to the
laboratories upon demand to the CNES International Polar Year
team on hrs_ipy@cnes.fr
The Polar DALI webpage,
http://polardali.spotimage.fr:8092/IPY/dalisearch.aspx contains
the login entry box and is separated in three main parts:
1. General parameters.
2. Region of interest.
3. The output format. Note that the results can be
directly displayed under Google Earth.
A short tutorial is also available to facilitate the first image
requests.
Figure 5. South hemisphere Spot5 HRS coverage achieved so
far (as of 17 April 2008) during the 2007-2008 austral summer
acquisition campaign.
The details of both campaigns are given in the Table 6 and 7,
which gives the area covered for each priority level in each
hemisphere and the number of attempts (60*120 km scenes).
Once the laboratory has investigated the catalogue and selected
its desired images, he can fill an order form to launch the
production of a SPIRIT DTM over his area of interest, if his
application is accepted by CNES. The SPIRIT product is
delivered through ftp.
2.4 The SPIRIT Product
Priority
Area of
interest
(km²)
Covered
area
(km²)
Coverage
rate
Attempts
number
1
179 925
2
187 615
3
312 608
Total
679 518
122 965
91 647
87 885
302 497
68.58%
48.85%
28.11%
44.52%
140
123
46
309
The SPIRIT product has been specifically designed to serve
glaciology applications, providing reference topography and
ortho-image. It is composed of 2 digital terrain models (DTMs)
computed using different sets of correlation parameters adapted
to different type of relief, the 2 reliability masks (one for each
DTM) and one 5-m ortho-image. The SPIRIT product is derived
from a single-date stereoscopic pair only at the French mapping
institute (IGN).
The DTMs has the following properties:
1. A 40-m posting interval.
2. The DTM is “no hole” (non-correlated pixels have
been interpolated).
3. The DTM is delivered under a DIMAP Geotiff format.
4. The absolute horizontal precision is estimated to be
30 m rms (Bouillon et al., 2006).
Table 6. Coverage achieved so far during the first SPIRIT
collection campaigns in the north (July to October 2007)
hemisphere.
Priority
Area of
interest
(km²)
Covered
area
(km²)
Coverage
rate
Attempts
number
1
330 040
2
596 844
3
989 384
Total
1 916 268
272 240
523 650
683 565
1 479 455
82.49%
87.74%
69.09%
77.21%
288
811
1 013
2 112
The reliability masks assume the following criterions:
1. They indicate the correlation score during DTM
generation from 0 to 100.
2. Interpolated pixels are reported with a score of 0 so
that they can be easily identified in the DTM.
3. The masks are point-to-point superimposable with the
corresponding DTMs.
Finally, the ortho-image presents the following characteristics:
1. Computed from one of the HRS images.
2. 5-m resolution.
3. Same absolute horizontal precision as the DTMs (30
m rms).
Table 7. Coverage achieved so far during the first SPIRIT
collection campaigns in the south (December 2007 to April
2008) hemisphere.
Thus, as of 17 April 2008, 1262 stereoscopic pairs have been
collected by SPOT 5 HRS. All the quick-looks are available for
The SPIRIT DTMs result of a predefined, automatic processing
method developed at IGN (Bouillon et al., 2006), including no
manual intervention and no check against any kind of ground
truth. Fusion between DTMs from different dates is not
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included. The original HRS stereoscopic pair (raw data) is not
delivered.
Joughin, I., Abdalati, W. and Fahnestock, M., 2004. Large
fluctuations in speed on Greenland's Jakobshavn Isbrae glacier.
Nature, 432(7017): 608-610.
3. CONCLUSION
Kaser, G., Cogley, J.G., Dyurgerov, M.B., Meier, M.F. and
Ohmura, A., 2006. Mass balance of glaciers and ice caps:
Consensus estimates for 1961-2004. Geophysical Research
Letters, 33(19).
During the international polar year, the SPIRIT project (SPOT 5
stereoscopic survey of Polar Ice: Reference Images and
Topographies) supported by CNES, Spot Image and LEGOS
allows building a massive high-resolution archive of Spot5 HRS
images. With the contribution of the French Mapping Institute
(IGN), DTMs and ortho-images will be generated from these
pairs of stereoscopic images for selected target areas and will be
delivered for free to the scientific community. The SPIRIT
project will thus contribute to improve our knowledge of the
topography of the polar ice masses. Together with other IPY
satellite acquisitions coordinated by the GIIPSY program,
SPIRIT will permit to build an IPY snapshot of the poles in
order to assess recent and future evolution of the polar
cryosphere.
In the coming months, our efforts to validate SPIRIT data will
go on. In the Northern hemisphere, we will use the ICESAT 3I
laser period that was active just a few weeks after the summer
2007 SPIRIT campaign. We will also analyze DTMs generated
from HRS stereo-pairs acquired on the less-textured landscapes
of Antarctica to test whether the promising result obtained in
the Northern Hemisphere are confirmed. We also invite all
users of the SPIRIT products to assess and report on its
accuracy through comparison with their own data derived from
other space-borne sensors or collected in the field during IPY.
Legresy, B. et al., 2005. ENVISAT radar altimeter
measurements over continental surfaces and ice caps using the
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Parrenin, F., Remy, F., Ritz, C., Siegert, M.J. and Jouzel, J.,
2004. New modeling of the Vostok ice flow line and
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Rabus, B., Eineder, M., Roth, A. and Bamler, R., 2003. The
shuttle radar topography mission - a new class of digital
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of Photogrammetry and Remote Sensing, 57(4): 241-262.
Raup, B.H., Kieffer, H.H., Hare, T.M. and Kargel, J.S., 2000.
Generation of data acquisition requests for the ASTER satellite
instrument for monitoring a globally distributed target: Glaciers.
IEEE Transactions on Geoscience and Remote Sensing, 38(2):
1105-1112.
Remy, F. and Parrenin, F., 2004. Snow accumulation variability
and random walk: how to interpret changes of surface elevation
in Antarctica. Earth and Planetary Science Letters, 227(3-4):
273-280.
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ACKNOWLEDGMENTS
We would like also to thank Ted Scambos (NSIDC, Colorado)
for sharing his MODIS mosaic of Antarctica (MOA) and
Greenland, Eric Rignot for sharing his ice velocity mosaic of
Antarctica and Greenland, D. Korn and T. Haran (NSIDC,
Colorado) for their help with ICESat data. Julian Dowdeswell
(SPRI, Cambridge) helped us to define SPIRIT targets in the
Arctic. Glacier outlines from the Digital Chart of the world
were kindly provided by Bruce Raup (NISDC). E.B. was
supported by the TOSCA “TOP GLACES API” proposal. We
acknowledge all the sponsoring agencies (ICSU, WMO) of the
International Polar Year and the framework provided by the
GIIPSY project. The CNES is acknowledged for the complete
funding of the SPIRIT project.
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